Is Exterior Insulation an Option?
Hi GBA Community,
New build in Zone 5 and I’ve come across the various resources on GBA as well as Fine Homebuilding discussing that the exterior insulation needs to keep the exterior warm enough to prevent moisture accumulation. Looking at the IRC R806.5 table that seems to now be R-20 for Zone 5.
I had planned on using 1/2″ Polyiso, house wrap and plywood sheathing on the my 2″x6″ exterior walls until I started doing my research and discovered that 1/2″ anything closed cell like XPS or Polyiso would do more harm than good. On the other hand, I also saw that Rockwool Comfortboard 80 is a great option since it is vapor-permeable but unfortunately they only start at a 1″ thickness.
At this point, due to the minimum thickness requirement of R-7.5 (from R702.7.1 – Class III vapor retarders) should I assume that exterior insulation is not an option for me?
Note: I could move my sills in another 1/2″ to gain that extra space to get a full 1″ of insulation, but then I’d have to get the blueprints updated and its also still less than the minimum R-7.5 and the recommended R-20.
Any advice? Thanks, Jason
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Replies
Consider foam on the interior side. And an interior side air barrier.
It's not being "closed cell" that is a potential issue, it's how vapor open the material is. A good example here is EPS, which is relatively vapor open as rigid foam goes, but EPS usually has a facer that makes the final product a vapor barrier. What you don't want is a vapor barrier on BOTH sides of the wall. If your wall allows inward drying (which is usually the case), then exterior rigid foam is not a problem, and will help make the wall more robust.
1/2" is too thin. The rules go by R value, not insulation thickness, so try not to focus on just the thickness of the rigid foam. 1.5" polyiso (a bit shy of R9) would be a good minimum for you here. Anything thicker is better. The warmer your wall, the less risk of moisture accumulation you have. A interior side smart vapor RETARDER (MemBrain, Intello, etc.) will also help to keep your wall safe.
I'm gradually working to get 3" of exterior polyiso on my own home, which will put about 60% of the the total R value of my 2x4 walls on the exterior. My goal is to be able to run a bit higher indoor humidity levels in the winter without risk to the walls.
Bill
Jon & Bill,
So I understand, if I can't achieve the recommended minimum value of R 7.5 (let's say for the sake of this example -- R5) as long as I bump up to using a Class I or Class II vapor retarder that will keep the wall safe and allow it to dry to the inside?
Kinda sorta... If you use the required minimum exterior rigid foam, the inside of the wall stays warmer, so dryer. If you use a vapor retarder on the interior side, you limit the amount of moisture that gets into the wall, so you make the wall safer. The vapor retarder permits drying, so the wall doesn't just get wetter and wetter over time. You really don't even need the interior side vapor retarder if you use enough exterior rigid foam, but the interior vapor retarder will always add an additional level of protection and increase the robustness of the overall wall assembly.
To allow "drying to the interior", you need to avoid any vapor barrier materials on the interior side. That means no polyethylene sheet, but it also means a few other things that you might not have thought of like no vinyl wallpaper. If you stick with painted drywall you should be fine.
Bill
Thanks Bill - that was extremely clear and helpful. Do you put a vapor retarder in every room of the house or only those rooms that would produce a lot of humidity (bathroom, kitchen).
Sidenote: my house is planning for radiant heat, so not sure if that helps/hurts anything. Thought it may help since we are not talking about as much humidity in the air compared to another system.